KVM: x86/mmu: simplify kvm_tdp_mmu_map flow when guest has to retry

A removed SPTE is never present, hence the "if" in kvm_tdp_mmu_map
only fails in the exact same conditions that the earlier loop
tested in order to issue a  "break". So, instead of checking twice the
condition (upper level SPTEs could not be created or was frozen), just
exit the loop with a goto---the usual poor-man C replacement for RAII
early returns.

While at it, do not use the "ret" variable for return values of
functions that do not return a RET_PF_* enum.  This is clearer
and also makes it possible to initialize ret to RET_PF_RETRY.

Suggested-by: Robert Hoo <robert.hu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -1159,7 +1159,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
  struct kvm *kvm = vcpu->kvm;
  struct tdp_iter iter;
  struct kvm_mmu_page *sp;
- int ret;
+ int ret = RET_PF_RETRY;
 
  kvm_mmu_hugepage_adjust(vcpu, fault);
 
@@ -1168,23 +1168,25 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
  rcu_read_lock();
 
  tdp_mmu_for_each_pte(iter, mmu, fault->gfn, fault->gfn + 1) {
+ int r;
+
  if (fault->nx_huge_page_workaround_enabled)
  disallowed_hugepage_adjust(fault, iter.old_spte, iter.level);
 
  if (iter.level == fault->goal_level)
  break;
 
- /* Step down into the lower level page table if it exists. */
- if (is_shadow_present_pte(iter.old_spte) &&
- !is_large_pte(iter.old_spte))
- continue;
-
  /*
  * If SPTE has been frozen by another thread, just give up and
  * retry, avoiding unnecessary page table allocation and free.
  */
  if (is_removed_spte(iter.old_spte))
- break;
+ goto retry;
+
+ /* Step down into the lower level page table if it exists. */
+ if (is_shadow_present_pte(iter.old_spte) &&
+ !is_large_pte(iter.old_spte))
+ continue;
 
  /*
  * The SPTE is either non-present or points to a huge page that
@@ -1196,13 +1198,17 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
  sp->nx_huge_page_disallowed = fault->huge_page_disallowed;
 
  if (is_shadow_present_pte(iter.old_spte))
- ret = tdp_mmu_split_huge_page(kvm, &iter, sp, true);
+ r = tdp_mmu_split_huge_page(kvm, &iter, sp, true);
  else
- ret = tdp_mmu_link_sp(kvm, &iter, sp, true);
+ r = tdp_mmu_link_sp(kvm, &iter, sp, true);
 
- if (ret) {
+ /*
+ * Also force the guest to retry the access if the upper level SPTEs
+ * aren't in place.
+ */
+ if (r) {
  tdp_mmu_free_sp(sp);
- break;
+ goto retry;
  }
 
  if (fault->huge_page_disallowed &&
@@ -1213,18 +1219,10 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
  }
  }
 
- /*
- * Force the guest to retry the access if the upper level SPTEs aren't
- * in place, or if the target leaf SPTE is frozen by another CPU.
- */
- if (iter.level != fault->goal_level || is_removed_spte(iter.old_spte)) {
- rcu_read_unlock();
- return RET_PF_RETRY;
- }
-
  ret = tdp_mmu_map_handle_target_level(vcpu, fault, &iter);
- rcu_read_unlock();
 
+retry:
+ rcu_read_unlock();
  return ret;
 }